Various methods and apparatus are used today to coat the interior surfaces of cylindrical objects, such as, for example, metal cans and containers used in the food and beverage industries. These coatings are typically applied in liquid form by spraying from an airless or air assisted spray nozzle through an open end of the container. Some spraying methods use more than one spray nozzle. The coatings are used to protect the food and drink contents of the container from the metal.
Such containers are typically two piece or three piece. In a two piece can, the can body may be a deep drawn cylinder with a closed end. A second piece is applied in a final operation to close the can after the contents have been added. In a three piece can, the cylindrical body or shell is open ended at both ends and further includes separate top and bottom end discs. The bottom end disc may be added prior to the coating operation much like a two piece can, and then the top end disc added to close the can after the contents have been added. However, double open ended can bodies or shells may also undergo separate coating operations, for example with a single nozzle at one open end or two nozzles at one open end or a single nozzle at each open end.
The inside surface profile or contour of such cans may vary from being completely smooth, to including contoured portions such as ribs for example or other contours or shapes. Also, the shape and profile of the bottom end disc may present additional challenges in assuring that the entire interior surface is adequately coated, particularly at the seam between the cylindrical sidewall and the outer edge of the bottom end disc. These complexities have led to a variety of spray nozzles to produce spray patterns that will result in adequate coating while attempting to minimize overspray. Overspray may be observed either as excessive coating material being applied to assure adequate coverage, loss of overspray material outside the can body when attempting to coat near the can body exterior edges, or both.
Typical spray patterns used for coating can interior surfaces are flat fan patterns. These flat fan spray patterns may include symmetrical spray patterns having maximum flow of the coating material in the middle with gradual diminished flows tapering or “feathering” from the middle to the ends of the pattern. Such a typical symmetrical pattern is illustrated herein with FIG. 2. Other flat fan spray patterns include asymmetrical or “controlled distribution” patterns such as produced by “drumhead” nozzles, in which the flow distribution is heavily skewed to one end or side of the spray pattern. Examples of nozzles used to produce flat fan spray patterns including drumhead nozzles are described in U.S. Pat. Nos. 3,640,758; 3,697,313; 3,726,711; 4,346,849 and 4,378,386, the entire disclosures all of which are fully incorporated herein by reference. These patents also provide excellent background information for various spraying methods and apparatus for single nozzles, two nozzles, spraying from one open end, spraying from two open ends and so on. These methods and apparatus are well known to those skilled in the art and therefore need not be repeated herein.
Spray coating processes for containers take into consideration two basic criteria. The first is the total weight of coating material used in the coating operation. The total weight significantly impacts overall cost. Total weight is a combination of overspray that is lost outside the container—for example when spraying near the outside edges of the side wall—and the quantity of material that is applied to the interior surfaces. The second important criteria is coating thickness and coverage. While adequate coating may be applied to assure sufficient thickness and coverage, from a cost and time standpoint it is wasteful to overcoat the container. Also, some surface areas may be more critical than others in specific applications and so it is not uncommon to overcoat less critical areas in order to assure adequate coverage of the critical areas. Coating processes are thus targeted to balance adequate coverage and coating thickness within a desired total weight goal, while trying to minimize overspray outside the container.